1. Field of the Invention
The present invention is directed to a transmission/reception circuit in a passive optical telecommunication system of the type having a central station in communication with a plurality of subscriber stations via respective optical waveguides.
2. Description of the Prior Art
Recent developments in telecommunications technology have lead to passive optical telecommunication systems wherein a plurality of decentralized stations (subscriber locations, or so-called distant units respectively combining a plurality of subscriber locations) are respectively connected via their own light waveguide subscriber line to an optical brancher. The optical brancher is connected directly or via at least one further optical brancher to a common light waveguide terminal of a central station--particularly an exchange--via a light waveguide bus. Such systems are described in European Application 0 171 080; "Passive Fiber Local Loop for Telephone with Broadband Upgrade," Oakley et al., ISSLS' 88, Conf. Papers pp. 9.4.1-9.4.5; and "The Provision of Telephony over Passive Optical networks," Hoppitt et al., BR Telecom Technol. J. Vol. 17 (1989) pp 100-113).
In such a passive optical telecommunication system, when the electro-optical transducers of the central equipment (exchange) and of the decentralized equipment (subscriber locations or distant units) preferably established by laser diodes transmit optical digital signals in the baseband with the same wavelength, i.e., in common-wavelength operation, the optical signals emitted by each location in such a telecommunication system can have an effect at that location's own receiver preferably established by a PIN-diode) due to reflections in the optical fiber network (optical cross talk), and thus can noticeably reduce the reception sensitivity thereof. Particularly critical is the approximately 1 through 10% reflectivity of laser diodes conventionally provided in decentralized equipment as electro-optical transducers. This is because measures for reducing reflection at laser diodes lead to a poorer coupling of the laser to the optical fiber, and thus to a lower optical power in the optical fiber, but parts of the light emitted by the central station are simultaneously reflected proceeding from all decentralized equipment to the opto-electrical transducer of the central station.
One can attempt to compensate for this problem by reducing the number of decentralized stations (subscriber stations (::lr distant units) per light waveguide terminal of the central station (exchange), and thus the demands made of the reception sensitivity of the central equipment are correspondingly reduced. This, however, means an increase in the number of light waveguide terminals in the central station and thus makes the entire system substantially more expensive, as would likewise the theoretically possible employment of optical isolators.